8 Summary and outlook

Transcription

1 91 8 Summary and outlook The main task of present work was to investigate the growth, the atomic and the electronic structures of Co oxide as well as Mn oxide films on Ag(001) by means of STM/STS at LT (100 K). The films were prepared by means of a reactive deposition of the metals in an O 2 atmosphere. The investigations were focused on the initial stage of the oxide film growth where a fascinating variety of different island types was found. In the first step, the Ag(001) substrate was investigated. The good quality of the used Ag(001) surface was confirmed by STM and LEED. Moreover, the electronic structure of the Ag(001) surface was determined by means of STS. A characteristic DOS peak was found in the (di/du)/(u/i) curves at an energy of ~1.6 ev above the Fermi level. In addition, a pronounces peak at 4.5 ± 0.1V was observed in the (di/du)/(u/i) spectra when the applied bias voltage ramp was extended to ± 5 V. This peak was assigned to the first FER peak. A systematic study of the field resonance states was performed by means of z(u) STS. The dependence of the number of the observed resonances on the applied bias voltage and on the tip displacement was studied. In addition, the tip/sample distance was estimated from I(z) spectroscopic measurements at bias voltages < 1V. The interaction of O 2 with Ag(001) was investigated at different substrate temperatures and different exposures. It was found that at 77 K O 2 is mainly molecularly adsorbed. At temperatures above 450 K, the formation of black dots related to O was observed. In addition, a shift of the Ag surface state peak in the (di/du)/(i/u) spectra was found. The presence of characteristic defects observed after several film growth experiments on Ag(001) surface was assigned to an O accumulation in the bulk followed by O segregation towards the surface. Co oxides on Ag(001) The deposition experiments were conducted at temperatures between 380 K and 500 K. The O 2 pressure was varied in the range between 10-6 mbar and 10-5 mbar. In the initial stage of growth, two types of Co oxide precursor develop (thickness 1 ML). In addition, 2-4 ML thick CoO(001) islands were found at an O 2 pressure of 10-6 mbar. The transition process from the precursor state to the double layer CoO(001) islands was characterised by STM studies. Increasing the O 2 pressure to 10-5 mbar led to the formation of (111) oriented Co oxide islands with a thickness of 1 ML and 2 ML. The latter islands showed a moiré pattern due to the formation of two (111) layers of CoO with slightly different lattice parameter. Finally, a phase diagram was drawn which displays the development of the different Co oxide species depending on the substrate temperature and the O 2 pressure. Using LT STM, the stability of the tunneling process was increased. This enabled high resolution STM imaging of the Co oxide islands and the Ag(001) surface. Atomic resolution was obtained for all oxide species allowing a structural characterisation of the islands.

2 92 Due to the high stability at LT, additional knowledge was gained concerning the voltage dependent contrast behaviour of all Co oxide island types. The measured apparent island height was drawn vs. the applied bias voltage. Different behaviour were observed for both precursor islands, the CoO(001) islands, and the (111) oriented Co oxide islands revealing their different electronic natures. In the bias voltage range of ± 5 V the 2 ML, 3 ML, and 4 ML thick CoO(001) islands did not show differences in their electronic behaviour. However, a contrast reversal (negative apparent island height) was only observed for 2 ML and 3 ML thick islands. A band gap of ev was estimated from the measurements. In addition, the influence of the first FER state on the apparent island height was studied. The data were found to be in agreement with STS investigations performed on the Ag(001) surface. STS investigations were successfully performed on all types of Co oxide islands. Spectroscopic fingerprints were obtained for each oxide species observed. A qualitative interpretation of the STS data was attempted and several models were developed in order to explain the peaks in the (di/du)/(i/u) curves. DOS effects as well as effects induced by resonant tunneling were assumed. The I(U) spectra as well as the (di/du)/(i/u) curves of the CoO(001) islands were analysed in more detail. The dependence of those spectra on the CoO island thickness was studied and no pronounced changes were found between 2 ML, 3 ML, and 4 ML thick islands. Additionally, a specific edge effect which influences the I(U) and (di/du)/(i/u) spectra was observed. This edge effect can be neglected if the spectra were taken only from the central part of the islands. The interaction of CoO(001)/Ag(001) with O 2 was studied. After annealing the sample in an O 2 atmosphere, a structural reordering of the islands was found. Characteristic shifts of the peaks in the (di/du)/(i/u) spectra towards higher energies were measured for both CoO islands and Ag(001). These shifts were observed for both W and Pt-Ir tips and were attributed by O. In addition, Co clusters were deposited onto CoO(001) films and studied by means of STM/STS. It was found that at RT Co mainly nucleates on uncovered Ag(001) areas and barely on the Co oxide islands. Depositing Co onto continuous and completely covering CoO(001) films, a large density of Co clusters is observed on the film terraces if the sample was cooled down to 77 K. This was different to RT deposition where step decoration occurs. The supported clusters showed an insulating behaviour as well as the tendency to form a band gap. However, the contribution of insulation support (CoO(001)) could not be clearly separated from those of the clusters. Mn oxides on Ag(001) First STM/STS investigations were performed on Mn film growth on Ag(001). The RT formation of a Mn-Ag surface alloy was analysed. A c(2x2) reconstruction was found in the LEED pattern for 1-2 ML thick Mn films on Ag(001). Surface alloying was also confirmed by STS since different peaks were found in the (di/du)/(i/u) spectra for Mn islands, mixed Mn-Ag areas, and Ag(001). Chemically selective STM imaging based on the locally varying work function was applied. A pronounced contrast difference was found in the STM images taken at 4.5 V and 4.8 V. Thus, it was possible to conclude that Mn islands

3 Summary and outlook 93 are surrounded by a Ag fringe whereas the underlaying substrate contains intermixed Mn-Ag areas. In addition, small Ag islands were found which probably appeared due to the exchange of Mn and Ag atoms in the initial growth stage. Depositing Mn in an O 2 atmosphere, at least six different island types were found strongly depending on the preparation conditions (O 2 pressure, substrate temperature). The STS data revealed clear differences between all islands. Some similarities to the CoO(001) islands were recognised. However, in this beginning stage of investigations, no clear conclusions on the geometric and electronic structures of the Mn oxide islands could be derived. Outlook To extend the knowledge of the electronic structure of the CoO(001) films, it is necessary to perform a series of z(u) experiments in order to study the FER formation in front of the CoO surface. This should help to separate the resonance effects from those caused by the DOS. An application of lock-in technique is also valuable for increasing the quality of the spectra. As a further step in the STS study, a spin polarised (SP) STS is of great interest. Thus, using SP STM, the antiferromagnetic nature of the TMO layers can be investigated.

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5 95 Abbreviations AED AES AREELS ARUPS BIS DFT DOS DRAM EELS et al. etc. e.g. i.e. IPE ISS fcc L LEED LSDA ML MRAM SIC STM STS, I/U/z UPS UHV XPS XRD Auger Electron Diffraction Auger Electron Spectroscopy Angle-Resolved EELS Angle-Resolved UPS Bremsstahlungisochromat Spectroscopy Density Functional Theory Density Of States Dynamic Random Access Memory Electron Energy Loss Spectroscopy et alii (and others) etcetera (and so on) exempli gratia (for example) id est (that is, in other words) Inverse Potoemission Ion Scattering Spectroscopy face centred cubic Langmuir Low Energy Electron Diffraction Local Spin Density Approximation Monolayer Magnetic Random Access Memory Self-Interaction-Correlation Scanning Tunneling Microscopy Scanning Tunneling Spectroscopy Current/Voltage/distance Ultraviolet Potoelectron/Photoemission Spectroscopy Ultra High Vacuum X-ray Photoelectron Spectroscopy X-Ray Diffraction

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